@WeiPingT if you want your child cases to be resolved automatically on parent case resolving, you have a setting in your parent case type resolve stage. -> Resolved all child cases. 

If you want to do it manually, you can get child cases keys(pzinskeys) from pxcoverinskeys from parent case clipboard, then loop pxcoverinskeys and obj-open each of the child cases and do pxchangestage to resolve stage. You can also use pxForceCase activity. But first approach is the better approach.